The present invention relates to radiant electrical heaters and particularly to heating elements therefor capable of high temperature operation.
The prior art is replete with attempts to increase the operating temperature capabilities of radiant electrical heating elements applied to a wide range of applications from cathode heaters for vacuum tubes to heaters for large industrial ovens. In many of these applications, elevating the operating temperatures would provide operating efficiencies and other benefits. One radiant electrical heating application of particular interest to applicants is in large portable ovens, such as disclosed in commonly assigned, copending application Ser. No. 491,509, filed May 4, 1983, for releasing the shrink fit of the wheels on a turbine rotor shaft. In such a procedure, it is necessary to inject large quantities of heat into the hub portion of the turbine wheel quickly so that the heel hub is heated at a much faster rate than the rotor shaft. The shrink fit can thus be released in less time while reducing the amount of energy wasted in heating the rotor shaft. To achieve this, the electric heaters are disposed as close as possible to the wheel, in some cases as close as one-quarter inch, while taking care to prevent them from actually contacting the wheel. Thus radiant heat transfer is the predominant heating mode. With such close spacings and the higher voltages requisite to achieving heating element operating temperatures above 2,000 degrees Fahrenheit, spurious electrical faults have proven to be a significant problem. That is, arcing from the heating elements to the wheel reduces heating efficiency and, if allowed to persist, will destroy the heating elements, and damage the workpiece being heated.
In an effort to increase the operating temperature capabilities of electrical heating elements, one approach has been to imbed or pot the resistance wire in various insulating compounds. While providing ample electrical insulation, potting has the distinct disadvantage of introducing excessive heat transfer resistance. Another approach has been to apply insulative coatings of various compositions to the resistance heating wire. Such prior art coating attempts have universally failed to satisfy all of the requirements for ultra-high temperature operation which are, inter alia, sufficient electrical resistance to prevent ground faults, sufficiently low thermal resistance so as not to degrade heater operation, adequate tenacity so as not to spall off after repeated thermal cycling, and practical in terms of material cost and the amount of labor, time and equipment to apply the coating.
It is accordingly an object of the present invention to provide an improved radiant electrical heating element.
An additional object is to provide a radiant electrical heating element of the above-character which is capable of operating at extremely high temperatures.
Another object is to provide a radiant electrical heating element of the above-character which is adapted for highly efficient radiant heat transfer.
Yet another object is to provide a radiant electrical heating element wherein the resistance wire thereof is adequately electrically insulated such as to inhibit spurious electrical ground faults to adjacent objects, including the workpiece to be heated.
A further object is to provide a radiant electrical heating element of the above-character which is reliable and efficient in operation, capable of withstanding repeated thermal cycling over a long operating life, and economical to manufacture.
Other objects of the invention will in part be obvious and in part appear hereinafter.